Ternary gas mixture which can be used in the braze welding of galvanized parts

ABSTRACT

A method of brazing galvanized metal parts such as those used for motor vehicle elements or container components. The parts are joined together to create a brazed joint by using at least one electric arc, a metal filler wire, and a gas shield. The gas shield contains hydrogen, carbon dioxide, and argon.

The present invention relates to a ternary gas mixture that can be usedin the braze-welding of galvanized parts.

Currently, the problems encountered when welding thin galvanized sheetare essentially due to the characteristics of the sheet.

This is because the thickness of the coated sheet used, especially inthe automobile industry, is usually between 0.5 mm and 1.5 mm.

Thicknesses as small as this require the welding process used to weldsuch sheet to be adapted so as to reduce the energy supplied andconsequently to avoid defects such as excessive penetration of the weldwith the risk of perforating the sheet, thermal deformation of thesheet, degradation of the zinc coating alongside and on the underside,or metallurgical and chemical degradation of the sheet.

Moreover, zinc, which is the main constituent of the coating on thingalvanized sheet, is characterized by a melting point of 420° C.(boiling point of zinc: 906° C.), which is lower than that of the basemetal and the filler metal.

During welding, it is therefore vaporized by the action of the electricarc or by simple thermal conduction, and this zinc vapor can then causeperturbations.

Thus, vaporized zinc may enter the atmosphere of the electric arc andsuddenly modify the physical properties of the shielding atmosphere,especially the electrical and thermal conductivity, and consequently cancause instabilities in the mode of metal transfer.

In addition, this vaporization of zinc in the molten metal may causemolten metal spatter on either side of the weld bead.

Furthermore, pores or blisters, which may form when zinc vaporizesbeneath the bead root when a lap weld is being made, often result in agas overpressure beneath the weld pool. This is the case the smaller thegap between the sheets to be joined together and the greater the zincthickness.

If the weld pool is cooled too quickly, the vapor does not have enoughtime to rise to the surface and, depending on its density, may affectthe mechanical properties of the joint.

The pores that open on the surface of the bead pose other problems, inparticular when painting a part thus welded, for example when this partconstitutes a visible portion of the body of a motor vehicle.

To try to solve these problems, the technique of braze-welding using aflame, MIG/MAG, TIG, plasma or laser process constitutes a goodalternative that has been used for many years in the automobileindustry.

Braze-welding makes use of copper-based filler metals having a lowermelting point, typically between 890° C. and 1080° C., than that of theferrous base metal of which the parts to be welded are composed, butabove that of the zinc coating, i.e. about 420° C.

In braze-welding, the parts to be welded together are joined not bymelting of the base metal and of the metal, but by “wetting” of thesolid base metal by liquid copper-based metal provided in the form of afiller wire.

Braze-welding requires a substantially lower injection of energy sincethis energy serves only to melt the filler wire and not to heat and meltthe part. Consequently, the amount of zinc vaporized is greatly reducedcompared with conventional welding.

The reader may refer to the document “The welding of galvanized steeland zinc-rich-painted steel”, Philips Welding Reporter, 1966, pages 1-10for further details about the braze-welding process.

Several types of alloy may be applied in braze-welding. CuAl(CuAl₈)-type alloys are used mainly for obtaining a nice-looking beadand good mechanical properties, whereas CuSi₃-type alloys are usedmainly for their more attractive costs and the ease of grinding off anyprojections.

In MIG braze-welding, metal transfer can take place in short-arc orpulsed mode.

Short-arc transfer is used for applications requiring a minimalinjection of energy and for “attractive” beads with no projections andwith limited deformation. Such transfer is limited to a certain wirespeed, to which a given welding speed range will correspond.

Pulsed transfer is used for applications with a higher wire speed andwelding speed than in short-arc welding. It leads to a hotter weld pool,which affords advantages in terms of joint clearance tolerance andwetting. The pulsed current complies with the rule of one drop perpulse, which drop must become detached during a background current(I_(background)) that is as low and as stable as possible.

Moreover, as is known, the shielding gas used in a MIG/MAG process playsan important role in the process as it has an appreciable influence onthe electrical and thermal properties of the electric arc atmosphere onthe one hand, and on the shielding of the weld pool.

As explained above, braze-welding is used with the objective of reducingthe injection of energy. Gases that are rather inert and substantiallyinactive will therefore be used.

The gas generally recommended for braze-welding is pure argon.

However, other gases or gas mixtures have already been described asbeing able to be used in brazed welding.

Thus, binary mixtures of argon with small amounts of oxygen or carbondioxide are known, for example from the following documents: MIG Lötenverzinkter Dünnbleche und Profile, Schweiβen und Schneiden, [MIG weldingof galvanized sheet and strip: Welding and cutting] by H. Hackl, 6.1998,Düsseldorf 1998; MIG Lötverbindungen, Besonderheiten und Eigenschaften,[MIG welded joints, special properties and characteristics], by H.Herold, DVS Report 204, Düsseldorf 1999; and a paper on MSGImpulslichtbogenschweissen von unbeschichteten und beschichtetenFeinbleche, [MSG pulsed-arc welding of coated and uncoated thin sheet]by G. Groten, DVS Volume 35, ISF Dissertation Aachen 1991, Dusseldorf1991.

However, other documents conclude that the presence of hydrogen isineffective. In this regard, mention may be made of the document by A.Kersche and S. Trube entitled Schutzgase zum Löten, Neue Technologienfür den Dünnblechbereich, [Shielding gases in welding: new technologiesfor the thin sheet field], SLV Munich 2000, which teaches that thepresence of small amounts of hydrogen causes porosity and poor wetting.

Furthermore, the document by Hauck and G. Hiller entitledLichtbogenschweissen verzinkter Stahlbleche, [Pulsed-arc welding ofgalvanized steel sheet], DVS Report 105, Düsseldorf 1986 is also known,which describes a mixture of argon with 30 vol % of helium in the caseof CuAl₈ wire and with 5 vol % oxygen in the case of CuSi₃ wire.

The present invention therefore aims to improve the MIG braze-weldingprocess by proposing a gas mixture that makes it possible, when it isused for the braze-welding of coated sheet, to obtain:

-   -   a reduction in the amount of energy injected, in order to reduce        the volume of volatilized zinc and deformations;    -   stabilization of the arc in order to prevent spatter; and    -   good wetting and good bead compactness in all welding positions,        with no porosity.

The solution of the invention relates to a ternary gas mixtureconsisting of hydrogen, carbon dioxide and argon in the followingproportions by volume:

-   -   0.4 to 2% hydrogen;    -   0.3 to 2% carbon dioxide; and    -   argon for the remainder (up to 100%).

Depending on the case, the gas mixture of the invention may comprise oneor more of the following technical features:

-   -   it contains at most 1.95% hydrogen, preferably at most 1.5%        hydrogen and even more preferably at most 1.3% hydrogen;    -   it contains at least 0.5% hydrogen, preferably at least 0.7%        hydrogen;    -   it contains at most 1% carbon dioxide, preferably at most 0.8%        carbon dioxide;    -   it contains at least 0.35% carbon dioxide, preferably at least        0.4% carbon dioxide;    -   it contains: 0.8 to 1.1% hydrogen; 0.4 to 0.7% carbon dioxide;        and argon for the remainder (up to 100%).    -   it contains approximately: 1% hydrogen; 0.5% carbon dioxide; and        argon for the remainder (up to 100%); and    -   the gas shield consists of a ternary mixture formed exclusively        from argon, hydrogen and carbon dioxide. However, inevitable        impurities may be found in the mixture in small proportions, for        example up to 20 ppm by volume of oxygen, up to 20 ppm of        nitrogen, up to 50 ppm of C_(n)H_(m) and up to 30 ppm of water        vapor.

The invention also relates to a process for the braze-welding ofgalvanized metal parts, in which a brazed joint is produced between theparts to be joined together by fusion, by means of at least one electricarc, a metal filler wire and the use of a gas shield for the braze,characterized in that the gas shield is formed from a gas mixture asgiven above. Depending on the case, the braze-welding process of theinvention may include one or more of the following technical features:

-   -   the parts to be joined together have a thickness of less than 3        mm, preferably between 0.5 and 2 mm and even more preferably        between 0.6 and 1.5 mm;    -   the filler metal is made of a copper aluminum alloy (CuAl alloy)        or a copper silicon alloy (CuSi alloy);    -   the metal parts are made of nonalloy carbon steel, preferably an        HYS (High Yield Strength) or VHYS (Very High Yield Strength)        steel;    -   the intensity of the current used to generate the arc or arcs is        less than 200 A for a wire of 1 mm in diameter;    -   the current is of variable or nonvariable polarity;    -   the transfer mode is of the pulsed or short-arc type;    -   the parts are galvanized by zinc electroplating or hot-dip        galvanized; and    -   it uses one or two filler wires.

The invention also relates to a process for manufacturing motor vehicleelements formed from several parts joined together by a braze-weldingprocess according to the invention, in particular of motor vehicleelements chosen from the group formed by the body of the vehicle or thefloor/hinge-knuckle joints, the engine cradle, the dashboardcross-members, the longitudinal members, the cross-members under theseats and the hydroformed components.

According to another aspect, the invention also relates to a process formanufacturing a container formed from several parts joined together by abraze-welding process according to the invention.

The braze-welding process of the invention may also be used to jointogether parts serving for the manufacture of other structures, such asglasshouse frames or the like, ventilation ducts, electrical cabinets,etc.

EXAMPLES

To demonstrate the feasibility and effectiveness of the gas mixtureaccording to the invention when it is used in braze-welding, comparativetrials were carried out under the following conditions.

The assemblies to be welded were formed from DX54D+Z120 sheets 0.8 mmand 1.5 mm in thickness according to the EN10142 standard and hot-dipgalvanized on both sides, that is to say having two zinc faces 10 μm inthickness.

The assemblies were positioned flat in a lap configuration. CuAl₈-typeand CuSi₃-type filler wires 1 mm and 1.2 mm in diameter were useddepending on the two types—short-arc and pulsed—of transfer.

The influence of the nature of the shielding gas was evaluated usingpure argon as control.

In the tests, the following evaluation criteria were used:

-   -   no spatter,    -   no open porosity (verification of closed porosity by        radiography);    -   good wetting: flat bead, little penetration, join angle;    -   no metallurgical defects: cracking, grain coarsening;    -   little degradation of the zinc coating; and    -   bead appearance: few deposits of adherent oxides.

For the purpose of expelling the zinc vapor upstream of the weld pooland preventing them from entering the gaseous atmosphere shielding theelectric arc, it is preferable in the MIG process to weld in the pushingdirection with a torch inclination of about 25°. Below this angle, zincvapor extraction is not as effective, which is manifested by arcinstability and spatter. Above this angle of 25°, perturbations occurwhen the molten metal is expelled by the gas jet. Moreover, the gas jetwith a horizontal orientation runs the risk of bringing in ambient airbehind the arc, which degrades the shielding of the weld pool.

The ideal gas flow for shielding the arc and the weld pool is given by avalue normalized to the area of about 0.05 l/min×mm². Thus, for a nozzle20 mm in diameter, in automatic welding, the flow rate is 30 l/min,whereas for a nozzle 16 mm in diameter, in manual welding, the flow rateis 20 l/min.

Example 1 Choice of Gas Mixture

Firstly, the inventors of the present invention have sought to determinethe effects of several gaseous compounds contained in an argon-basedshielding gas mixture.

The components of the mixture that were tested and the results obtainedare given in Table I below.

TABLE I Arc Compactness/ Other Gas stability Porosity Wetting Oxidationrisks Argon + 0 0 + CO₂ +++ + − − O₂ +++ − + −− Root porosity H₂ ++ − ++−− He 0 + ++ + Flared arc N₂ + 0 0 + In Table I: “+++” means excellent;“++” means very good; “+” means good; “0” means moderate; “−” meanspoor; and “−−” means very poor.

As regards arc stability, Table I shows that, when oxidizing components,such as O₂ and CO₂, are added to the argon the arc stability isincreased through the formation of surface oxides that are moreemissive. Nitrogen may also bring about a stabilizing effect, but to alesser extent.

Certain components, such as He or H₂, in argon have a positivecontribution to the appearance and morphology of the bead.

In helium, the arc requires a higher voltage and therefore a largeamount of energy injected into the weld pool, which may improve thebead-wetting conditions but penetration control is made more difficulton thin sheet.

Hydrogen helps to improve the morphology and the appearance of the bead.

The first property is due to an arc constriction effect in the zoneclose to the end of the wire, that is to say an endothermic dissociationthat causes substantial cooling of the external periphery, and thereforethe constriction, and a substantial heat recovery effect at theworkpiece to be welded, namely a recombination on the surface with therelease of energy.

The second property, through the reducing effect of hydrogen, allowsbeads to be obtained that are free of surface oxides.

Example 2 Testing of the 98.5% Ar/1% H₂/0.5% CO₂ Gas Mixture

In this example, the gas mixture mixture formed from 98.5 vol % Ar+1 vol% H₂+0.5 vol % CO₂ was assessed in automatic welding and in manualwelding, using the following parameters:

-   -   In automatic welding:        -   nozzle-workpiece distance (d): 15 mm        -   gas flow rate (Q): 30 l/min        -   welding speed (Vw): 50 cm/min        -   torch angle to the vertical: 25°    -   In manual welding:        -   nozzle-workpiece distance (d): 12 mm        -   gas flow rate (Q): 20 l/min        -   welding speed (Vw): 40 cm/min        -   torch angle to the vertical: 25°

The other parameters adopted (type of filler wire, transfer mode, etc.)are given in Table II below for the 98.5% Ar/1% H₂/0.5% CO₂ mixture.

TABLE II V_(wire) U_(mean) I_(mean) V_(w) P_(mean) E_(lin) [m/min] [V][A] [cm/min] [kW] [J/mm] Pulsed CuAl₈ transfer 0.8 mm 4 19 72 75 1.37110 (1) (17.6) 1.5 mm 5 19.4 95 50 1.94 233 (17.6) CuSi₃ 0.8 mm 2.7 1944 50 0.84 101 1.5 mm 5 20.2 86 50 1.74 209 E_(lin) V_(wire) U_(mean)I_(mean) V_(w) (3) P_(mean) (3) [m/min] [V] [A] [cm/min] [kW] [J/mm]Short- CuAl₈ arc 0.8 mm 3.6 15.6 87 ≈70 1.36 116 transfer 1.5 mm 4.616.3 106 ≈70 1.73 148 (2) CuSi₃ 0.8 mm 3.7 14.9 79 ≈70 1.18 101 1.5 mm5.5 15.2 106 ≈70 1.61 138 (1) Tests carried out in automatic weldingwith a 480TR16 current generator sold by La Soudure Autogène Francaise;(2) Tests carried out in manual welding with a TPS2700 current generatorsold by Fronius; (3) Estimated manual welding speed as an indicator.

In the U_(mean) column, the numbers in brackets correspond to thevoltage in the case of pure argon.

The results obtained are compared as regards the metallurgicalappearance and the operating conditions, namely ease of implementation,melting of the wire, arc stability and amount of spatter.

FIG. 1 makes it possible to compare arc stability in pulsed welding withargon (top graph) with, for comparison, the Ar/H₂/CO₂ gas mixture of theinvention (bottom graph). As may be seen, in pulsed welding theimprovement in arc stability is manifested by less dispersion in thepeak voltage (U) and, at low voltages, in the drop detachment voltage.

In short-arc welding, it is essentially the uniformity of the short-arcfrequency, the arc time/short-arc time ratio and the short-arc currentthat are appreciated.

Wetting is of particular interest in braze-welding.

FIG. 2 makes it possible to compare the wetting obtained in pulsedwelding with argon (top graph) with, for comparison, the Ar/H₂/CO₂ gasmixture of the invention (bottom graph).

In the macrographic sections in FIG. 2, the wetting is characterized bythe width (L), the thickness (H), the penetration (P) and the join angle(a). An example of this evaluation is appended (FIG. 2).

The mechanical performance of the joints obtained with sheet 0.8 mm inthickness is assessed by the tensile strength (R_(m)) measured in a“guided” tensile test.

The results are given in Table III below for the ternary Ar/CO₂/O₂ gasmixture of the invention (composition: 98.5% Ar; 1% H₂; 0.5% CO₂) and,for comparison, a binary gas mixture consisting of argon to which 2 vol%of CO₂ has been added (i.e. Ar/2% CO₂).

TABLE III Max. V_(w) Heat/ R_(m) (3) Wire/gas [m/min] Spatter AppearanceWetting Oxides Deformation [MPa] Pulsed CuAl₈ transfer (1) mixture+++ + + + + 304.4 Argon + + 0 0 + 296.8 Ar/H₂/N₂ ++ − 0 0 − 303.0 CuSi₃Mixture +++ + + + + 272.3 Ar + 2% CO₂ ++ 0 0 0 + 275.4 Ar/CO₂/O₂ 0 0 0 −−− 271.4 Short-arc CuAl₈ transfer (2) Gas + ++ ++ ++ +++ − mixture Argon0 + + + ++ − CuSi₃ Ar + 2% CO₂ 0 0 0 0 ++ − Mixture + ++ ++ 0 +++ − (1)and (2): see legend beneath Table II; (3) Systematic fracture in thebase metal; Assessment rating: see legend under Table II.

As may be seen, the ternary gas mixture of the invention (98.5% Ar+1%H₂+0.5% CO₂) results, by comparison, in very good results compared withthe binary control mixture.

Additional tests carried out in some have shown that, in the range from0.3 to 2 vol % CO₂, a satisfactory arc stabilization effect is obtainedfor a CO₂ content of about 0.5%. However, it is not desirable toincrease this CO₂ content as it results in negative effects as regardsoxidation of the welded joint, especially oxide deposits and smoke.

Likewise, in the range from 0.5 to 10% H₂, the upper limit is determinedby the maximum solubility of hydrogen in the molten metal, which ismanifested by the risk of porosity appearing as soon as the upper limitis exceeded. By means of a multilayer deposit, which simulates rework,local distribution or consolidation, it has been shown that this riskdoes not exist in the case of a hydrogen concentration of less thanabout 2%. For safety, the H₂ content has been limited to 1 vol %, whichalso represents a good compromise between improving the shape andappearance of the bead, and the increase in welding speed. Totalhydrogen assays in the deposited metal have revealed a content of 6 μ/g,which corresponds to 0.067 g/cm³, knowing that the maximum solubilityvalue acceptable from the industrial standpoint is 0.100 g/cm³ in thecase of a copper aluminum alloy.

One specific criterion in the automobile industry is the capability ofthe process to absorb gaps between the sheets, that is to say assemblytolerance. For thin workpieces (thickness <1.5 mm), the gap may be equalto the thickness.

Particularly advantageously, the joints welded with the ternary gasmixture of the invention have tolerated a gap ranging up to 2 mm in thecase of a 1.5 mm thickness, i.e. a gap greater than the thickness of thewelded parts.

By way of comparison, the use of gas mixtures based on argon to which afew percent of oxygen or nitrogen has been added results in greaterenergy being injected into the weld pool and rapidly entails a risk ofundesirable puncture of the top sheet of the assembly.

Furthermore, when braze-welding, it is desirable to create a joint withminimal dilution of the base metal, typically iron. However, slightfusion of this base metal is tolerated so as to prevent simple bondingof the bead.

During the tests carried out within the context of the presentinvention, it was observed that the degree of dilution of the iron inthe deposited metal, calculated according to volume contributions,remains below 5% in the case of sheet 1.5 mm in thickness, which is avalue that is quite acceptable from the industrial standpoint.

The gas mixture of the invention has furthermore shown an ability toachieve good workability in manual welding since vertical downhandwelding and horizontal-vertical welding do not require particularparameters to be adapted compared with flat-position welding.

It should be noted that the surface appearance of the bead may befurther improved using a “drag rod” that extends the shielding zone byabout 50 mm in order to optimize the gaseous shielding of the weld poolduring cooling, as this makes it possible to eliminate the copper oxidesformed on the bead, a phenomenon taking place essentially with the CuSi₃filler metal.

If the annular nozzle is fed with the ternary shielding gas mixture ofthe invention, the “drag rod” may preferably be fed with the same gasmixture or with pure argon (flow rate: about 10 l/min).

1. A method for braze-welding galvanized metal parts comprisingproducing a brazed joint between said parts to be joined together,wherein said joint is produced with a brazing means, wherein said meanscomprises: a) at least one electric arc; b) at least one copper aluminumalloy filler wire; and c) a gas mixture, wherein said gas mixtureconsists essentially of: 1) about 0.4% to about 2% hydrogen by volume;2) about 0.3% to about 2% carbon dioxide by volume; and 3) about 96% toabout 99.3% argon.
 2. The method of claim 1, wherein said gas mixturecomprises less than about 1.95% hydrogen.
 3. The method of claim 2,wherein said gas mixture comprises less than about 1.5% hydrogen.
 4. Themethod of claim 3, wherein said gas mixture comprises less than about1.3% hydrogen.
 5. The method of claim 1, wherein said gas mixturecomprises at least about 0.5% hydrogen.
 6. The method of claim 5,wherein said gas mixture comprises at least about 0.7% hydrogen.
 7. Themethod of claim 1, wherein said gas mixture comprises less than about 1%carbon dioxide.
 8. The method of claim 7, wherein said gas mixturecomprises less than about 0.8% carbon dioxide.
 9. The method of claim 1,wherein said gas mixture comprises at least about 0.35% carbon dioxide.10. The method of claim 9, wherein said gas mixture comprises at leastabout 0.4% carbon dioxide.
 11. The method of claim 1, wherein said gasmixture consists essentially of: a) about 0.8% to about 1.1% hydrogen;b) about 0.4% to about 0.7% carbon dioxide; and c) about 98.2% to about98.8% argon.
 12. The method of claim 1, wherein said gas mixtureconsists essentially of: a) about 1% hydrogen; b) about 0.5% carbondioxide; and c) about 98.5% argon.
 13. The method of claim 1, whereinsaid parts to be joined have a thickness of less than about 3 mm. 14.The method of claim 13, wherein said parts to be joined have a thicknessof about 0.5 mm to about 2 mm.
 15. The method of claim 1, wherein saidparts to be joined together are motor vehicle elements comprising atleast one member selected from the group consisting of: a) vehiclebodies; b) floor/hinge-knuckle joints; c) engine cradles; d) dashboardcross-members; e) longitudinal members; f) cross-members under seats;and g) hydroformed components.
 16. The method of claim 1, wherein saidparts to be joined together further comprise parts made of non-alloycarbon steel.